Using Curl in PHP for bitbucket APIs

I'm trying to learn curl in PHP, I tried implementing bitbucket API which has following syntax for authentication:

$ curl -X POST -u "client_id:secret" \
  https://bitbucket.org/site/oauth2/access_token -d grant_type=password \
  -d username={username} -d password={password}

This is as per the documentation: https://confluence.atlassian.com/bitbucket/oauth-on-bitbucket-cloud-238027431.html Which while using in PHP I did something like this:

$postData = array(
    'grant_type' => 'password',
    'username' => '*******',
    'password' => '**********'
);
$curl = curl_init();
curl_setopt_array($curl, array(
    CURLOPT_URL => 'https://bitbucket.org/site/oauth2/access_token',
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_MAXREDIRS => 10,
    CURLOPT_TIMEOUT => 30,
    CURLOPT_CUSTOMREQUEST => "POST",
    CURLOPT_POSTFIELDS => $postData,
));

$response = curl_exec($curl);

But I'm getting an error

"{"error_description": "Client credentials missing; this request needs to be authenticated with the OAuth client id and secret", "error": "unauthorized_client"}"

I tried using client_id and secret too like this:

$postData = array(
    'grant_type' => 'password',
    'client_id' => '*******',
    'secret' => '**********'
);

But still no help.